1. Field of the Invention
The present invention generally relates to an ATM (Asynchronous Transfer Mode) cell transfer apparatus. More specifically, the present invention relates to an ATM cell transfer apparatus suitable for transferring an OAM (Operation And Maintenance) cell.
2. Description of the Related Art
A processing method of an OAM cell executed in an ATM cell transferring operation is defined in the specification I.610 of ITU-T (International Telecommunication Union—Telecommunication Standardization Sector). In this specification, a processing time required to transfer an OAM cell when any failure occurs is also defined. Further, an OAM cell transferring operation in PVC (Permanent Virtual Channel) is defined.
A processing operation of an OAM cell should be carried out as follows in this specification to notify the occurrence of any failure. That is, it is supposed that a receiving apparatus operates as a terminal point of either an end-end connection or a segment connection in a F4 (VP: Virtual Path) flow or a F5 (VC: Virtual Channel) flow while a cell is received. In this case, when an AIS (Alarm Indication Signal) OAM cell for either the F4(VP) flow or the F5(VC) flow is transmitted from a counter apparatus, the receiving apparatus must send back an RDI (Remote Defect Indication) OAM cell within a time period of 500 ms or transfer the AIS OAM cell in the time interval of one second in accordance with the connection.
FIG. 15 shows a definition of a terminal point of an end-end connection of an F4(VP) flow or an F5(VC) flow, or a terminal point of a segment connection of the F4(VP) flow or the F5(VC) flow. Referring to FIG. 15, when a PVC connection is actually set between apparatuses, an ATM cell signal flows through the connection in an F4(VP) flow or an F5(VC) flow. In this case, a connection point for terminating the ATM cell signal is defined as an “end point”. A connection point which is arbitrarily defined by an operator of an apparatus on an end-end connection is defined as a “segment point”. FIG. 15 represents these definition states. It could be considered that a segment connection point may be defined as a node point in the end-end connection or as one end node point in the end-end connection.
As types of OAM cells, there are AIS, RDI, Loop Back, Continuity Check, Performance Monitoring (PM), and so on. All of these OAM cells other than a PM OAM cell must be sent back to a counter apparatus within a certain time period, when they are received.
FIG. 14 shows a processing operation of an OAM cell. Referring now to FIG. 14, the processing operation will be described. Here, it is supposed that an ATM cell is transferred through a physical link layer from a left direction to a right direction in this drawing. In this case, the transfer direction of the ATM cell is referred to as a “signal forward direction”, whereas the direction opposite to the transfer direction of the ATM cell is referred to as a “signal backward direction”. For example, in case that any failure such as an LOS or an LOF (Loss Of Frame) occurs in the physical link layer, an RDI OAM cell is generated, and then is sent out in the signal backward direction.
Also, an AIS OAM cell is generated in case that any failure such as interruption of signal input, any error on a transfer path, and LOF occurs in a port for terminating a transfer path or an interface with another connection point. As described above, in case that any abnormal state is caused in the F4(VP) flow or the F5(VC) flow so that the AIS OAM cell is received, the AIS OAM cell is detected in an SDH/SONET layer as a physical layer. At this time, the AIS OAM cell is transferred in the signal forward direction. In addition thereto, an RDI OAM cell is sent back to the signal backward direction.
As above described, when the failure occurs, the sending back of the RDI OAM cell or the generation and transfer of the AIS OAM cell at the connection relaying point need to be carried out for each of the end-end connection and the segment connection with respect to a plurality of F4(VP) flows within the above port, and a plurality of F5(VC) flows within each of the plurality of F4(VP) flows.
FIG. 1 shows the structure of a conventional ATM cell transfer apparatus. A switch block is put between physical interfaces. Transfer paths on which SDH/SONET signals are transferred are connected to each of the physical interfaces. A CPU is connected to the switch block.
As shown in FIG. 1, the generating and transferring operations of the OAM cell is carried out by the CPU in the conventional ATM cell transfer apparatus. Every time an ATM cell is received, the CPU analyzes the content of the received ATM cell. Then, the CPU carries out the sending back operation of the RDI OAM cell in the F4(VP) flow or the F5(VC) flow, and the transferring operation of the AIS OAM cell based on the analyzed content. However, it is sometimes difficult that the CPU carries out all of the processing operations of the OAM cells.
Of the above described OAM cells, the OAM cell of Loop Back and the OAM cell of Continuity Check need to be sent back only when an on-demand is issued from a counter apparatus. As a result, these OAM cells of Loop Back/Continuity Check are not frequently generated or sent back while the actual ATM service operation is carried out. Therefore, the generating and sending back operations of these OAM cells may be satisfactorily carried out even by the processing operation of the CPU.
However, as shown in FIG. 14, there is a problem in the processing operation of the AIS/RDI OAM cell. As types of the processing operation, there are two types of the end-end connection and the segment connection, also there are cases that a plurality of F4(VP) flows are present in a transfer port in STM-1 and OC-3c, and furthermore, a plurality of F5(VC) flows are present in each of the F4(VP) flows.
As above described, when any failure occurs in a plurality of transfer paths or ports which have a large number of F4(VP) flows or F5(VC) flows in the PVC connection, a large amount of AIS OAM cells need to be generated within one time. In the case that all of the processing operations are carried out in a software manner by the CPU, such a large amount of AIS OAM cells can not be generated at a time as defined in the above-described I.610 specification of ITU-T. This is because the CPU determines the OAM cells one by one in accordance with a software program as to whether the OAM cells should be generated. Therefore, even when the OAM cells should be generated by the CPU, the generation timing thereof may be deviated. In the worst case, there is a possibility that this CPU itself may stop the processing operation thereof. Also, there is another possibilities that a similar unfavorable result may be caused, when a large number of AIS OAM cells are transferred from the counter apparatus in a large number of F4(VP) flows or F5(VC) flows.
To solve these problems, Japanese Laid Open Patent Application (JP-A-Heisei 9-36869) is known. In this reference, an OAM cell is not transferred based on the software processing operation carried out by the CPU, but the generation of an RDI OAM cell and the detection of the occurrence of any failure on a transfer path is carried out by a hardware circuit in response to the reception of an AIS OAM cell. Then, the generated OAM cell is sent out on a VP having a VPI (Virtual Path Identifier) value specified by a CPU.
In this reference, as shown in FIG. 2, an OAM cell is transferred from the counter apparatus and is detected by an OAM cell detecting unit (1). Then, the OAM cell having a VPI value indicative of a transfer destination is stored in a buffer (9). Also, the AIS OAM cell which should be generated when any failure occurs on the transfer path is stored into the buffer (9) after the value of the VPI register (4) is set by the CPU. The OAM cells stored in the buffer (9) are finally sent out from the OAM cell transmitting unit (10).
In this reference, the CPU does not determine and send out all of the OAM cells which should be transferred to the counter apparatus based on the given data such as input OAM cell and alarm information.
However, as the types of the OAM cells transferred from the counter apparatus, not only the VPI value but also the VC value should be considered. Also, in this conventional ATM cell transfer system, the manipulating operations for the respective OAM cells on the end-end connection or the segment connection are not considered. Further, this conventional. ATM cell transfer system does not consider which type of OAM cell should be inputted, and whether or not an OAM cell corresponding, to the inputted OAM cell should be sent back. In addition, this conventional ATM cell transfer system similarly does not consider the manipulating operation of the OAM cell when the failure occurs on the transfer path.
Moreover, in the above described conventional ATM cell transfer apparatus, in case that a plurality of F5(VC) flows are present in an F4(VP) flow, if the transferred OAM cell is the OAM cell in the F4(VP) flow, the OAM cell can not be transferred for the F5(VC) flows within the F4(VP) flow.
In conjunction with the above description, a cell output apparatus is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 4-363939). This conventional cell output apparatus is provided with a buffer (3) for storing the cell of a transfer data and buffers (4 and 5) for storing the cell having a characteristic pattern. A buffer selecting apparatus (6) refers to an output priority order reference table (14) to select any one of these buffers (3, 4 and 5). The cell stored in the selected buffer is transferred from a cell transmitting apparatus (7). In this manner, the cell output apparatus is capable of changing the data pattern of a physical layer OAM cell and the output priority order of the physical layer OAM cell without expanding the scale of the hardware.
Also, an OAM processing method at a plurality of line terminating apparatus is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 10-262064). In this conventional OAM processing method, a physical layer terminating unit (30) is connected to a plurality of lines to convert reception signals into cells. The converted cell is added at the header with a line identifier which specifies a line on which the converted cell is transferred. A header converting unit (34) converts the line identifier VPI/VCI value of the cell into an internal processing ICID. An OAM processing unit (37) collectively manages the data required for carrying out the OAM processing operations for the respective lines by using an internal processing connection management table which uses the internal processing identifier as an address. The data is read out from the management table based on the ICID of the cell received from each of the lines, and then the OAM processing operation corresponding to the line is carried out based on the read data. In this way, the plurality of lines are commonly controlled by the single OAM processing unit.
Also, an OAM cell inserting apparatus is disclosed in Japanese Patent No. 2746284. In this conventional OAM cell inserting apparatus, normal data cells are stored in a cell memory unit (30). An OAM cell is generated in response to an OAM cell output instruction before a total number of cells stored in the memory cell unit (30) is reached to a predetermined value. Thus, either of the data cell or the OAM cell is outputted. As a result, the OAM cell is outputted from the OAM inserting apparatus without any delay, if necessary.
Also, an operation maintenance cell sending circuit is disclosed in Japanese Patent No. 2851941. In this conventional cell sending circuit, a line interface unit (3) is connected to the line and a switch unit (7), and contains a physical layer terminating unit (4) and a VPI converting unit (5). Furthermore, the VPI converting unit (5) is composed of an operation maintenance sending circuit (6).